The use of magnetizable particles affixed to various substrates to record, store, and reuse information continues to increase. The various forms of magnetizable particles affixed to a substrate, often termed magnetic media, include, audio tapes, video tapes, computer diskettes, computer tapes, data cartridges and many other products. In each of these constructions, magnetic particles are bound to a substrate and information is recorded and stored in the magnetizable particles for later retrieval and use.
Magnetic media must possess certain physical and magnetic properties to be suitable for use with the various magnetic reading and recording devices. The surface of the media must be sufficiently smooth so that when read by the magnetic head the recorded signal is read accurately. Asperities or roughness in the media surface can result in unacceptable levels of signal to noise ratio. The media must also be very durable. It must be able to record and reproduce information repeatedly, preferably thousands, if not millions, of times. To be durable, the magnetic particles must be firmly bound to the substrate and not be worn off by the passing of the magnetic head over the media. In addition, it is important that the media have sufficient lubricity so that the magnetic head passes freely over the surface with a minimum coefficient of friction, preferably without the necessity of added lubricants. Another important characteristic of a magnetic media is that it have very uniform physical properties over a wide temperature range, such as a range of 100.degree. C.
Typically, a magnetic media is produced by passing a non-magnetic support through an apparatus which coats the support with a liquid dispersion of the magnetic layer. This dispersion consists of a binder, in either an uncured or solvated state, having the magnetizable particles homogeneously dispersed therein. After coating, the dispersion dries or cures leaving a tough binder film having the magnetizable particles uniformly distributed throughout.
The desired physical and magnetic properties are often dictated by certain processing parameters. One important parameter is good pigment wetting in the bulk liquid dispersion. Good pigment wetting is closely related to dispersion stability and both are necessary to insure evenly distributed pigment throughout the dispersion and throughout the finished magnetic coating. Another important processing parameter is the so-called pot life of the dispersion, the time for which the dispersion can be kept sufficiently non-viscous before coating. In general terms, a short pot-life, caused by a fast cure of the binder, creates time constraints in the coating process and problems in obtaining a sufficiently smooth surface on the magnetic layer. A longer pot-life often requires a slower cure rate, often resulting in the coated magnetic layer suffering damage as it is passed through the coating apparatus in its insufficiently cured state.
A majority of conventional magnetic media binders are derived from lower molecular weight materials which require curing to generate a binder resin having the appropriate physical properties. In addition to the pot-life considerations discussed above, the use of curable low molecular weight starting materials can cause additional problems. The dispersion of the low molecular weight materials may prematurely gel or cause flocculation of the magnetic particles resulting in non-uniform and unacceptable magnetic performance of the coatings. In addition, low molecular weight materials may remain in the binder after coating resulting in poor durability of the magnetic coating. Further, low molecular weight materials left after curing may migrate to the surface of the media, and come into contact with the recording head where they can adversely affect performance through increased friction, stiction, head clogging, and/or poor blocking resistance.
Other problems encountered by magnetic media using conventional curable binder systems known in the prior art include hydrolysis of the binder and degradation of the binder by humidity. In addition, the activation or cure steps required by conventional binder systems with curable lower molecular weight starting materials create additional complicated and expensive production steps.
Known polymeric binder systems experience a change in physical properties over a range of temperatures. In use, magnetic media are often exposed to a wide range of temperatures, often over a range in excess of 100.degree. C. The modulus of the binder material will typically substantially change over this range, for example, from stiff and brittle to soft and tacky. These changes in physical properties can cause performance problems in the use of the media.
In view of the above deficiencies in prior art magnetic media binders, there is a need to provide a binder suitable for use in magnetic media which is comprised of high molecular weight starting materials, which do not require polymerization or crosslinking to form a suitable magnetic coating. In addition, it is desirable that these high molecular weight binder materials provide good pigment wetting and dispersion stability, including systems with high pigment loadings. It is also desirable that the magnetic media have excellent mechanical properties, such as smoothness, durability and lubricity, along with excellent electrical properties, such as signal-to-noise ratio. It is also desirable that the physical and magnetic properties of the media are relatively uniform throughout a wide range of temperatures.